SAW components use acoustic waves which travel at the speed of sound. The SAW components are preferred over widely used transmission line components because acoustic waves have a substantially shorter wave length at operating frequency than electromagnetic waves which travel at the speed of light. Therefore, for a given operating frequency, a SAW filter provides a smaller structure than a transmission line structure, therefore, making them suitable for miniaturized radio frequency applications. Furthermore, SAW structures are easily integratable with other active circuits, such as amplifiers and mixers, which are produced using conventional integrated circuit technologies. For the above reasons, the popularity of SAW structures in radio frequency applications, especially in filter applications, has been increasing steadily.
SAW filters are particularly used in communication devices to provide selectivity at various stages of a receiver, such as at the front-end stage or at the IF stage of the receiver. The selectivity of a SAW filter is determined by its bandwidth which is defined as the frequency spectrum limited between the 3 dB points of the filter's frequency response.
When used in the IF stage of the receiver, the bandwidth of the IF SAW filter is dependent upon the type of modulation used in the receiver. With recent developments in telecommunication technology, particularly with the advent of the personal communication systems, such as cordless telephone second-generation also known as CT2, European and Japanese digital cordless telephone systems, respectively known as DECT and JDCT, more complex modulation techniques are used for communicating messages. These new digital services require wider bandwidths than usually used on conventional communication systems. For example, the CT2 system uses time division duplex (TDD) with frequency division multiple access (FDMA) technology which supports digital data rate of 72 kilobits per second per channel with a 32 kilobits per second adaptive differential pulse code modulated (ADPCM) voice signal. The CT2 system requires a 100 KHz IF bandwidth for communicating messages. On the other hand, the European digital cordless telephone system (DECT) utilize a time division multiple access (TDMA) system supporting digital data at 1152 kilobits per second per channel which uses a 32 kilobits per second ADPCM voice signal. The DECT requires approximately 1200 KHz of IF bandwidth, whereas, the Japanese digital cordless telephone system (JDCT) uses QPSK modulation at 384 kilobits per second per channel with 32 kilobits per second ADCPM voice which requires an IF bandwidth of 240 KHz. In the United States of America, several systems with different modulation and IF bandwidth requirements are under consideration. The modulations under consideration are GFSK or QPSK with IF bandwidths of approximately 400-500 KHz.
It is desirable to provide a universal IF circuit which is capable of servicing different modulation techniques and accommodate their bandwidth requirements accordingly. A universal IF circuit, in addition to providing a more flexible radio receiver, reduces design and manufacturing costs as well. However, conventional SAW filter designs provide a fixed bandwidth and are not suitable for applications requiring multiple bandwidths. Therefore, there exists a need for a multi-bandwidth SAW filter which may, for example, be used in a radio capable of operating in communication systems having different modulation requirements.